Saturable absorbers for q-switching of middle infrared laser cavaties

ABSTRACT

This disclosure demonstrates successfully using single, polycrystalline, hot pressed ceramic, and thin film Fe doped binary chalcogenides (such as ZnSe and ZnS) as saturable absorbing passive Q-switches. The method of producing polycrystalline ZnSe(S) yields fairly uniform distribution of dopant, large coefficients of absorption (5-50 cm −1 ) and low passive losses while being highly cost effective and easy to reproduce. Using these Fe 2+ :ZnSe crystals, stable Q-switched output was achieved with a low threshold and the best cavity configuration yielded 13 mJ/pulse single mode Q-switched output and 85 mJ in a multipulse regime.

PRIORITY CLAIM

This application is a continuation in part of U.S. patent applicationSer. No. 11/140,271 Entitled: “Mid-IR Microchip Laser: ZnS:Cr²⁺ Laserwith Saturable Absorber Material” filed May 27, 2005 which is adivisional of U.S. patent application Ser. No. 10/247,272 filed Sep. 19,2002, now U.S. Pat. No. 6,960,486, and also claims priority from U.S.Provisional Patent Application No. 60/863,268 filed Oct. 27, 2006.

FIELD OF THE INVENTION

The present invention relates to the field of lasers and materials usedin lasers. More particularly the present invention relates to materialsused in the output control of lasers, specifically saturable absorberswhich can be used for Q-switching of laser cavities. In even greaterparticularity the present invention relates to the use of Fe:ZnS andFe:ZnSe poly-crystalline structures in laser applications.

BACKGROUND

Certain medical or biomedical devices are based on Cr:Er:YSGG or Er:YAGlasers operating in free-running oscillation regime near the wavelengthof water absorption—2.8-2.9 μm. Lasers emitting in the 3 μm wavelengthregion are needed in the medical field as surgical tools. This use of alaser as a laser scalpel or drill is due to the absorption of water inthis spectral region. To effectively use such a laser, it must have highenergy as well as short pulses that can be provided by Q-switching of Erlaser cavities.

The temporal output of the current lasers is characterized by multiplespikes of ˜1 μs pulse duration spreading irregularly over the flashlampdischarge pulse of approximately 100-200 μs. The drawback of irregularcharacter of the spikes is that the spikes with energy below thethreshold of teeth ablation deposit their light energy towards teethheating, resulting in painful sensations that might appear in the teethof the patients.

To eliminate this problem and the need of anesthesia during treatment itis proposed to utilize passive Q-switched regime of Cr:Er:YSGG operationwith a much shorter (<150 ns) but regular multiple pulses each withenergy above the threshold of ablation to eliminate pain sensationswhile preserving cutting efficiency of the dental hard tissue.

The simplest way to obtain the required regime of ns multiple laserpulses with high peak powers in a cost-effective, compact and reliableall-solid-state laser system consists in laser cavity passiveQ-switching by inserting a saturable absorber inside the Cr:Er:YSGGresonator. However, commercial passive solid-state Q-switches for the 3μm spectral range are not currently available. A 2.94 μm Er:YAG laserwas Q-switched using a rotating, mirror as reported by Bagdasarov,Danilov et al, electro-optic Q-switch as reported by Bagdasarov, Zhekovet al, and a passive water and ethanol Q-switch as reported byVodopyanov. Successful realization of the 1.3-2.1 μm laser cavitiespassive Q-switching with the use of Cr doped ZnSe and ZnS crystals wasdemonstrated by several research groups. However, the use of Fe dopedchalcogenides for the passive Q-switching of laser cavities at longer2.4-3.4 μm spectral range was not evident and trivial due to a strongnon-radiative quenching of the excitation in these materials at roomtemperature. To characterize the effectiveness of Fe²⁺:ZnSe as asaturable absorber in the Mid IR Spectral region and as a potential gainmedium, the cross-section of absorption versus wavelengths must bemeasured. As one can see from FIG. 1 the absorption cross section ofFe²⁺ ion in the ZnSe crystal measured at λ=2.94 μm is ˜9.5×10⁻¹⁹ cm²,which is approximately 35 times higher than the cross section for thelaser transition of the Er³⁺ ion in yttrium-aluminum garnet. Thecombination of a high value of saturation cross-section, smallsaturation energy with good opto-mechanical and physical characteristicsof the ZnSe host (damage threshold—2 J/Cm², Knoop Hardness 1.20 kg/mm²,thermal conductivity 18 W/mK, dn/dT=70×10⁻⁶ K⁻¹) make Fe²⁺:ZnSe crystala promising material for passive Q-switching of mid-infrared lasercavities.

A significant problem is that growth of Fe doped ZnSe crystals is nottrivial. Bulk Fe²⁺:ZnSe crystals can be obtained from melt or vaporgrowing techniques by including the dopant in the starting charge. Underatmospheric pressure ZnSe is sublimed at a temperature higher than about1100° C. before melting. It is therefore for melt growth, in addition tohigh temperature (1525° C.), necessary to apply high pressure, up to75×10⁻⁵ Pa [6]. This inconvenience of the ZnSe high temperature meltgrowth might be accompanied by uncontrolled contamination inducingundesired absorptions. On the other hand, the control of the amount ofFe²⁺, ions incorporated in the crystal is difficult using vapor growthtechnique. Hence, utilization of other more cost effective methods ofFe²⁺:ZnSe fabrication is of interest.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a mechanism to generate nsduration multiple laser pulses with high peak powers in acost-effective, compact and reliable all-solid-state laser system. Thisobject can be accomplished using polycrystalline or single crystallineFe:ZnSe and Fe:ZnS as saturable absorbing passive Q-switches. The methodof producing polycrystalline and single crystalline ZnSe(S) yieldedfairly uniform distribution of dopant and large coefficients ofabsorption (5-50 cm⁻¹) while being highly cost effective and easy toreproduce. Using these polycrystalline Fe:ZnSe crystals, stableQ-switched output was achieved with a low threshold. The most optimalcavity configuration yielded 13 mJ/pulse single mode Q-switched outputand 85 mJ in a multi-pulse regime.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the absorption cross section of Fe²⁺ ions in ZnSe crystal

FIG. 2 is the absorption spectra of Fe²⁺:ZnSe and Fe²⁺:ZnS samples

FIGS. 3 a, b, & c are the temporal profiles of single pulse (3 a) outputand multi-pulse outputs (3 b,c) from the passively Q-switched Er:Cr:YSGGlaser system utilizing a Fe²⁺:ZnSe saturable absorber.

FIG. 4 shows Fe²⁺:ZnSe transmission versus incident 2.8 μm photon flux,with the solid line theoretical fit of experimental results withFrantz-Nodvick equation for σ=0.6×10⁻¹⁸ cm².

FIG. 5 is a schematic layout of a linear cavity design for Er laserQ-switch utilization of the Fe²⁺:ZnSe saturable absorber.

FIG. 6 is a schematic layout of a folded cavity design for Er laserQ-switch utilization of the Fe²⁺:ZnSe saturable absorber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In our experiments, the undoped polycrystalline and single crystallinesamples of ZnSe were grown by chemical vapor deposition. Doping of the1-3 mm thick ZnSe polycrystalline and single crystalline wafers wasperformed by after growth thermal diffusion of Fe from the metal or gasphase in quartz evacuated ampoules. Alternatively, Fe doped thin filmsof the ZnS and ZnSe the crystals were grown by pulsed laser depositionon ZnS/Se substrates. In addition, Fe:ZnS and ZnSe were fabricated byhot pressing of ZnS and ZnSe powders containing iron. Demirbis et alestimated the diffusion coefficient for iron and chromium ions to be7.95×10⁻¹⁰ cm²/s and 5.45×10⁻¹⁰ cm²/s, respectively at 1000° C. In ourpreparation, the sealed ampoules were placed in a furnace and annealedat 820-1120° C. for 5-14 days. Once removed from the furnace and cooled,doped crystals were extracted from the ampoules and polished. Thismethod of production of transition metal doped crystals is covered inU.S. Pat. No. 6,960,486 commonly owned by the assignee of thisapplication and which is incorporated by reference herein for allpurposes.

The Q-switched regime of operation for a Er:Cr:YSGG laser system has twodistinctive qualities: large amplitude pulses and temporally shortpulses with respect to free running oscillation. Both of these qualitiesare needed for medical applications as well as to ensure efficientQ-switched operation of Fe²⁺:ZnSe lasers at room temperature. Theabsorption spectra of Fe²⁺:ZnSe and Fe²⁺:ZnS ⁵E-->⁵T₂ transitions aredepicted in FIG. 2. These transitions feature a broad absorptioncentered at ˜3 μm with FWHM of approximately 1400 nm. Further, theabsence of exited state absorption makes polycrystalline Fe²⁺:ZnSe avery good candidate for a passive Q-switch for an Er laser.

In our experiments a flashlamp pumped Er:Cr:YSGG laser was used as atest bed for passive Q-switching. Many cavity designs were tested,however in all cavity designs the laser head includes a 73 mm longEr:Cr:YSGG crystal with a 3 mm diameter in a gold elliptical pumpingchamber pumped with a xenon flashlamp. FIG. 5 schematically illustratesa linear design with a 100% reflective mirror, HR, and an OC withreflectivity of 83% or 40%. The HR was placed approximately 70 mm fromthe end of the Er:Cr:YSGG laser crystal and the OC was placedapproximately 50 mm from the laser crystal. The Fe²⁺:ZnSe was sampleplaced between 17-65 mm from the high reflector in the cavity. The laserwas pulsed at 10 Hertz. Input power was determined by directly measuringthe voltage across the capacitor driving the flashlamp. The output wasmeasured with a Molectron EPM 1000 power meter or a JR-09 joule meter.For this cavity, at maximum pump energy of 31 J, an output energy of 0.5J was achieved in a free-running mode.

Using a 4×8×1 mm 90% initial transmission at 2.8 μm, Fe:ZnSe placed atthe Brewster angle Q-switched operation was achieved. We obtained singlegiant pulse lasing with a pulse duration of approximately 65 to 100 nsFWHM measured with a pyroelectric detector with a rise time ofapproximately 15 ns (See FIG. 3 a). A maximum output energy of 5 mJ for80% OC and approximately 7 J pump energy was achieved. The ratio ofenergy of single giant pulse to the respective free-running energyapproached 20% and could be further increased with improvements ofFe:ZnSe quality.

A multi-pulse regime was also obtained using either the 83% or the 40%OC, yielding multiple pulses depending on pump power although betterperformance was obtained using the 40% OC. The threshold for lasing withthis OC was approximately 9 J. The five pulse regime shown in FIG. 3 brepresents a nearly ideal train of pulses with little energy differencefrom pulse to pulse. The pump energy for five pulses was 14J.Multi-pulse output with a maximum of 19 pulses was obtained with 85 mJtotal output energy at pump energy of 30 J with a 40% OC as shown inFIG. 3 c. Utilization of a 50% initial transmission Fe:ZnSe sample,yielded 9 mJ output energy using a 40% OC and 42 J pump energy.

Altering the cavity to a folded cavity scheme using three mirrors andtwo output beams allows the effective reflectance of the OC to be tunedwith angle (see FIG. 6). Also this design reduced the photon flux uponthe Fe²⁺:ZnSe sample allowing a sample with a high initial transmissionto be more effectively used as a passive Q-switch with littledifficulty. The HR was located approximately 115 mm from the lasercrystal. The cavity was folded at approximately 45 degrees using a 40%reflecting OC as the folding mirror at approximately 180 mm from thefront of the laser crystal. A 82% reflecting mirror was used as thesecond HR. The Fe²⁺:ZnSe sample was placed on this side as a passiveQ-switch. The pulse repetition rate was reduced to 4 Hz to deal withthermal lensing problems. Using this setup enabled maximum Q-switchedsingle pulse energy of 13 mJ with 65 ns FWHM using 30 J of pump energy.Similar results on Cr:Er:YSGG cavity Q-switching were obtained with theuse of single thermally diffused Fe:ZnSe crystals as well as withhot-pressed ceramic Fe:ZnSe and thin films of Fe:ZnSe grown by pulsedlaser deposition. Thus we propose these Fe²⁺:ZnSe materials for use as apassive Q-switch, particularly for Er lasers.

Further, Fe²⁺:ZnS, having similar spectroscopic properties to Fe²⁺:ZnSe,is known to have the larger bandgap (3.84 vs. 2.83 eV), bettermechanical and optical damage characteristics, better overlap ofabsorption band with the Cr:Er:YSGG lasing wavelength, highercross-section of absorption at 2.8 μm, as well as lower thermal lasingdn/dT (+46×10⁻⁶ vs.+70×10⁻⁶/° C.). Therefore, the intracavity energy andpower handling capability of this material should lie higher; makingFe²⁺:ZnS very attractive for high energy, high power applications.Parallel experiments to those with Fe:ZnSe have been performed usingFe:ZnS, fabricated similarly to Fe:ZnSe by after growththermo-diffusion. A ˜5×8×1 mm sample of Fe²⁺:ZnS with an absorptioncoefficient of 6 cm⁻¹ and an initial transmission of 75% at 2.8 μm wasutilized as a passive Q-switch. Using a linear cavity design placing theFe²⁺:ZnS sample at the Brewster angle between the HR and Er:Cr:YSGGcrystal, with an 80% reflectance OC, Q-switching experiments wereperformed. Approximately 5 mJ per pulse was obtained. Similar results onCr:Er:YSGG cavity Q-switching were obtained with the use of singlethermally diffused Fe:ZnS crystals as well as with hot-pressed ceramicFe:ZnS and thin films of Fe:ZnS grown by pulsed laser deposition. Thuswe propose these Fe²⁺:ZnS materials for use as a passive Q-switch,particularly for Er lasers.

The Q-switched output of the Er:Cr:YSGG laser was used for saturationstudies of Fe:ZnSe. The saturation curve of Fe:ZnSe was measured (FIG.4). It's fitting with the Frantz-Nodvick equation results in, absorptioncross section of 0.6×10⁻¹⁸ cm², which is of the same order of magnitudeas the absorption cross-section obtained from spectroscopic measurements(1.0×10⁻¹⁸ cm². Hence, the described Fe-doped ZnSe and ZnS crystals arevery promising as passive Q-switches for mid-IR Er lasers operating overthe 2.5-4.0 μm spectral range.

Although the invention has been described in various embodiments it isnot so limited but rather enjoys the full scope of any claims grantedhereon.

1. A passive solid state saturable absorber selected from the groupconsisting of single and polycrystalline Fe²⁺:ZnSe and Fe²⁺:ZnS for useas a Q-Switch for 3 μm Erbium lasers.
 2. A Q-switch for Erbium laserscomprising a saturable absorber selected from the group consisting ofsingle and polycrystalline Fe²⁺:ZnSe and Fe²⁺:ZnS,
 3. A passivesaturable absorber selected from the group consisting of Fe dopedchalcogenides for use as a Q-Switch for Erbium lasers.
 4. A saturableabsorber as defined in claim 3 wherein said saturable absorber is formedby: a. forming a polycrystalline or single crystalline structure of athickness sufficient for use as a microchip saturable absorber, wherethe crystal is selected from the group of chalcogenides including ZnSand ZnSe, b. depositing a thin film layer of Fe on opposing faces ofsaid crystal by a method selected from the group of pulsed laserdeposition, cathode arc deposition, thermal evaporation, and plasmasputtering; c. annealing said crystal sealed in vacuumed ampoules in anoven for a period and at a temperature sufficient to allow crystaldoping by Fe diffusion and replacement in selected regions of saidcrystal.
 5. A saturable absorber as defined in claim 3 wherein saidsaturable absorber is formed by: a. forming a polycrystalline or singlecrystalline structure of a thickness sufficient for use as a microchipsaturable absorber, where the crystal is selected from the group ofchalcogenides, b. annealing said crystal sealed in vacuumed ampoulestogether with iron containing chemical in an oven for a period and at atemperature sufficient to allow crystal doping by Fe diffusion andreplacement in selected regions of said crystal.
 6. A saturable absorberas defined in claim 3, wherein said saturable absorber is formed by athin film of Fe doped chalcogenides grown by pulsed laser deposition,plasma sputtering, or thermal evaporation on the transparent at lasingwavelength substrate made from similar or dissimilar material.
 7. Asaturable absorber as defined in claim 3, wherein said saturableabsorber is fabricated by hot pressing of chalcogenide powderscontaining iron.
 8. An Er laser with resonator Q-switched at roomtemperature by a saturable absorber as defined in claim 1.